In catalytic reforming processes, the charge is generally a naphtha distilling for example between 60.degree. and 200.degree. C., in particular straight-run naphthas. Light naphthas are mostly removed by distillation, their reforming being of little interest since they cannot be converted to aromatics and are not much isomerized. These light naphthas are then either recovered as such for the gasoline pool, or upgraded by isomerization in a separate unit.
The isomerization of normal paraffins of low molecular weight is of primary importance in the oil industry, in view of the particularly high octane number of isoparaffins.
The conversion of C.sub.4 -C.sub.7 and mainly C.sub.5 -C.sub.6 n-paraffins to isoparaffins is interesting for improving light gasoline fractions, particularly the straight-run top fractions, in order to obtain a motor fuel of high octane number.
Naphthas, usually called heavy naphthas, are thus the only feed to the reforming unit.
In the process according to the invention, a reforming unit is used jointly with an isomerization unit to satisfy the new regulations for manufacturing lead-free gasoline, particularly in Europe and the U.S.A. For upgrading light naphtha cuts, it has been suggested to integrate the isomerization unit with the reforming unit in order to minimize the investment and the utilities requirements. Until recently, isomerization reactions were conducted with chlorine injection, using as a catalyst a carrier having a group VIII noble metal incorporated therewith, the carrier being for example impregnated with a hydrocarbylaluminum halide of the formula: EQU AlX.sub.y R.sub.(3-y)
wherein
y is 1 or 2 PA1 X is a halogen and PA1 R is a hydrocarbon monovalent radical (British patent 1,432,639)
Catalysts of the Friedel and Crafts type, such as aluminum chloride, can also be used. In addition to the conventional catalysts such as platinum on silica-alumina, halogenated alumina or other acid carriers, U.S. Pat. No. 3,903,192 indicates the use of alumina carriers with introduction into alumina of halogen and of at least one compound of metal from group VI or VIII of the periodic classification of elements, in the presence of a mixture of chlorine and/or hydrochloric acid.
All these catalysts may be used for isomerizing, in hydrogen atmosphere, paraffins containing 4 to 7 carbon atoms and preferably 5 and/or 6 carbon atoms, at a temperature from 50.degree. to 250.degree. C. The operation is preferably conducted under a pressure from 5 to 100 kg/cm.sup.2 (5-100 bars) with a space velocity from 0.2 to 10 liters of charge per liter of catalyst and per hour. It may be convenient to continuously or periodically introduce into the charge a halogenated promoter, for example hydrochloric acid, carbon tetrachloride, or an alkyl halide such for example as ethyl chloride, isopropyl chloride, tert-butyl chloride or tert-butyl bromide.